The invention relates to fabricating C/C composite material friction parts, particularly, but not exclusively, airplane brake disks.
Herein, the term fraction parts “based” on C/C composite material is used to mean friction parts made of C/C composite material or made essentially of C/C composite material, i.e. that may include small percentages by weight of additional elements, e.g. ceramic particles, in particular for the purpose of improving wear resistance.
Airplane brake disks based on C/C composite material are in widespread use. A well-known method of fabricating such disks comprises the following steps:                making an annular preform out of carbon-precursor fibers, typically pre-oxidized polyacylonitrile (PAN) fibers;        applying carbonization heat treatment to transform the carbon precursor and obtain an annular preform made of carbon fibers and intended to form the fibrous reinforcement of the composite material; and        densifying the carbon fiber preform with a carbon matrix.        
An annular preform of carbon precursor fibers may be made in various ways:                forming a thick fiber structure by superposing plies of two-dimensional fiber texture, bonding together the superposed plies, and cutting out annular preforms from the fiber structure, the two-dimensional fiber texture being for example a multidirectional (nD) fiber sheet obtained by superposing unidirectional (UD) fiber sheets and bonding the UD sheets together, e.g. by light needling;        cutting out annular plies or plies in the form of solid disks from a two-dimensional fiber texture, e.g. an nD sheet, and then superposing annular fiber plies and bonding the superposed plies together in order to obtain directly an annular fiber preform or a disk-shaped fiber preform from which the central portion is then cut out so as to obtain an annular preform; or        winding flat turns of a helical braid or fabric so as to form superposed annular fiber plies, and bonding the plies together.        
In those various processes, the bonding between the superposed plies is conventionally performed by needling. For this purpose, and typically, the superposed plies are placed on a horizontal support and needling is performed progressively as the plies are superposed on one another, with a needling pass being performed each time a new ply is added. The needling is performed by means of barbed needles that penetrate vertically (Z direction) into the fiber structure or fiber preform that is being formed, with bonding between plies being obtained by the fibers that are moved by the needles so that they occupy the Z direction. The horizontal support is caused to move down by one step each time a new ply is applied after a needling pass so as to control the density in the Z direction of fibers passing through the thickness of the fiber structure or the fiber preform.
Concerning the preparation of annular preforms made of carbon precursor fibers, reference may be made for example to the following documents: U.S. Pat. Nos. 4,790,052, 5,792,715, and 6,009,605.
It should be observed that making an annular preform out of carbon fibers directly by superposing carbon fiber plies and bonding those plies together by needling has also been proposed.
Prior to densifying with a PyC matrix, it is known to perform high temperature heat treatment on the carbon fiber preforms, typically at a temperature of at least 1600° C., in particular to eliminate any impurities contained in the fibers, in particular residual sodium stemming from the process for preparing carbon precursor fibers. By way of example, reference may be made to the following documents: U.S. Pat. Nos. 7,351,390, 7,052,643, and 7,410,630.
Densification by a carbon matrix may be achieved by a liquid-type process, namely by impregnating the preform with a carbon precursor in liquid state, such as a resin or pitch, and by transforming the precursor into carbon by carbonization under heat treatment.
The densification with a carbon matrix may also be performed by a chemical vapor infiltration (CVI) process comprising, in well-known manner, placing carbon fiber preforms in an enclosure and admitting into the enclosure a gas that contains one or more gaseous precursors of carbon, with the conditions, in particular of temperature and pressure, within the enclosure being controlled so as to enable the gas to diffuse within the preforms and form a PyC deposit therein by the precursor(s) decomposing. The gas typically comprises methane and/or propane as carbon precursor(s), it being understood that other gaseous hydrocarbon precursors could be used. A plurality of annular preforms placed in a stack may be densified simultaneously within a single enclosure, as described in particular in document U.S. Pat. No. 5,904,957.
It is also possible to perform densification with a PyC matrix using a “vaporization” process comprising, likewise in known manner, immersing an annular preform of carbon in a bath of a liquid carbon precursor, and heating the preform, e.g. by coupling with an induction coil. On contact with the heated preform, the liquid vaporizes. The vapor diffuses and generates a PyC deposit by decomposition within the preform. Reference may be made in particular to document U.S. Pat. No. 5,733,611.
It is also known to achieve densification by combining a CVI process and a liquid-type process. Documents EP 2 088 347 and EP 2 093 453 disclose a densification step by CVI followed by a densification step by impregnation with pitch and carbonization. Pitch carbonization is carried out at a temperature between 1200° C. and 1800° C., typically 1600° C. and may be followed by graphitization heat treatment at a temperature between 1600° C. and 2400° C. to graphitize the pitch-precursor carbon.
The present invention relates to the manufacture of friction parts based on C/C composite material in which the carbon of the matrix is formed of PyC originating from a precursor in gaseous state at least in a main external phase of the carbon matrix. By “PyC originating from a precursor in gaseous state” is meant here PyC obtained by a conventional CVI process as well as PyC obtained by the above mentioned vaporization process.
After densification with a PyC matrix, it is known optionally to proceed with final heat treatment at high temperature, typically higher than 2000° C., in order to graphitize the PyC matrix when it is of rough laminar type PyC or “RL-PyC”. Amongst the various types of PyC that may be obtained under the conditions in which the CVI process is performed (in particular isotropic PyC, smooth laminar PyC, RL-PyC), RL-PyC is the type that lends itself to graphitization. A process for preparing an RL-PyC matrix is described in document U.S. Pat. No. 6,001,419.
Airplane brake disks made of C/C composite material with an RL-PyC matrix graphitized by final heat treatment at high temperature (material “A”) presents good resistance to oxidation and gives good braking performance, in particular good stability of the friction coefficient during high energy braking such as emergency stop braking at high speed prior to takeoff, also known as rejected takeoff (RTO) braking. Nevertheless, the wear of such disks is relatively high.
Brake disks made of C/C composite material without final heat treatment at high temperature but with high temperature heat treatment performed on the carbon fiber precursor prior to densification (material “B”) presents low wear at low energy, in particular braking while taxiing when cold, where that constitutes a large component of the total wear usually observed during a normal operating cycle comprising taxiing while cold (including braking) from a parking point to takeoff, flight, braking on landing, and taxiing while hot (with braking) from the runway to a parking point. Nevertheless, compared with material A, lower resistance to oxidation and smaller stability of the friction coefficient during high energy braking have been observed.